Ceramic materials have several properties which are beneficial in bearing applications. Silicone nitride, for example, typically has a hardness twice that of bearing steel and is thus more wear-resistant. Ceramic materials are also lighter in weight and have a lower surface roughness than bearing steel, which is beneficial for reducing friction and energy losses. Bearings with ceramic components are therefore excellent in high-speed applications, such as in machine tools. In some examples, the bearing has steel bearing rings and one or more ceramic rolling elements. These bearing are known as hybrid bearings. In other examples, the bearing rings and the rolling elements are made of a ceramic material. Such full ceramic bearings are used, for example, in applications where weight must be minimized. Furthermore, ceramic materials are chemically inert and can withstand higher operating temperatures than bearing steel, which makes full ceramic bearings extremely suitable in aggressive operating environments.
A drawback of ceramic materials in bearing applications is that they are susceptible to surface crack initiated rolling contact fatigue. After manufacture, the tribological surface of e.g. a ceramic ball can exhibit some porosity, and may also contain inclusions and small surface cracks. Pores and inclusions can act as crack-initiation sites, and a surface crack which develops, or a pre-existing surface crack, may propagate under rolling contact fatigue, leading to bearing failure
Bearings which comprise one or more ceramic components are sometimes lubricated with solid lubricant coatings such as molybdenum disulfide or graphite.
There are also numerous applications where the bearing is more suitably lubricated with a non-solid lubricant like oil or grease. It has been found that in such applications, there is a particular need to prevent or inhibit crack propagation.